The present invention relates to a process for producing a purified aqueous hydrogen peroxide solution, and more particularly to a process for producing an aqueous hydrogen peroxide solution having a high purity, which enables the removal of silicon oxide impurities contained in an aqueous hydrogen peroxide solution with accuracy up to ppt order (1/1012).
An aqueous hydrogen peroxide solution is widely used in various fields, for example, for a bleaching agent for paper or pulp and as a component in chemical polishing fluids. In recent years, hydrogen peroxide has increasingly been used in the electronic industry, for example, as a cleaning agent for silicon wafers and as a cleaning agent in production processes of semiconductors. Accordingly, hydrogen peroxide requires a very high purity by extremely decreasing a variety of impurities in an aqueous hydrogen peroxide solution.
In general, hydrogen peroxide is produced almost exclusively by an anthraquinone process at present. The anthraquinone process is conducted as follows. A derivative of anthraquinone, such as a 2-alkylanthraquinone, is converted into the anthrahydroquinone by hydrogenation in a water-insoluble solvent in the presence of a hydrogenation catalyst. After the catalyst is removed, the reaction product is oxidized with air to regenerate the 2-alkylanthraquinone, and hydrogen peroxide is produced at the same time. By extracting the produced hydrogen peroxide from the oxidation with water, an aqueous solution containing hydrogen peroxide can be obtained. This process is called the anthraquinone auto-oxidation process.
The aqueous hydrogen peroxide solution produced by this process contains inorganic ions such as Al, Fe, Cr, Na, and Si, and inorganic compound impurities, which are derived from materials constituting the apparatus. Therefore, higher purity of an aqueous hydrogen peroxide solution is achieved by removing these impurities so as to meet a quality required.
As the process for purification of an aqueous hydrogen peroxide solution by removing those impurities such as inorganic ions and compounds contained in the aqueous hydrogen peroxide solution, for example, the process comprising bringing an aqueous hydrogen peroxide solution into contact with a strongly acidic cation exchange resin is known. When the strongly acidic cation exchange resin is thus used, cationic metal ion impurities such as Na, K and Ca can be easily removed. In the process described above, there remains trace amount of anionic impurities such as SO42xe2x88x92, and Al ion, Fe ion, Cr ion which sometimes form anionic metal chelate impurities Therefore, such anionic impurities and anionic metal chelate impurities are generally removed by contacting these impurities with the strongly acidic cation exchange resin.
However, in the above conventional processes, it is hardly possible to remove silicon oxide impurities contained in an aqueous hydrogen peroxide solution.
Therefore, many methods have been proposed to remove silicon oxide impurities contained in an aqueous hydrogen peroxide solution.
For example, Japanese Patent Application Laid-Open Publication No. 9(1997)-142812 discloses a process for removing silicon oxide impurities by bringing an aqueous hydrogen peroxide solution containing silicon oxide impurities into contact with an anion exchange resin in the fluoride ion form.
Japanese Patent Application Laid-Open Publication No. 9(1997)-221305 discloses a process for removing silicon oxide impurities by filtrating an aqueous hydrogen peroxide solution with an ultra-filter. It is described therein that, in the process, pH of hydrogen peroxide is preferably preset at 3 to 5 and the aqueous hydrogen peroxide solution may be further brought into contact with an ion exchange resin after an ultra-filtration process.
Japanese Patent Application Laid-Open Publication No. 9(1997)-235107 discloses a process wherein a fluoride compound is added to an aqueous hydrogen peroxide solution and the mixture is brought into contact firstly with a cation exchange resin in a H+ form having sulfonic acid group and secondly with an anion exchange resin.
Further, Japanese Patent Application Laid-Open Publication No. 11(1999)-79717 discloses a process for producing a purified aqueous hydrogen peroxide solution by bringing an aqueous hydrogen peroxide solution into contact with a strongly basic anion exchange resin in a hydroxide ion form after adding hydrogen fluoride of not less than 0.05 milliequivalent per liter of aqueous hydrogen peroxide solution to an aqueous hydrogen peroxide solution.
Other than the processes for purification described above, there are other processes for purification known, wherein some of RO (reverse osmolysis membrane) apparatus, a cation exchange resin tower, an anion exchange resin tower, a Mix bed (a mixed bed resin), an ultra-filtration apparatus, a chelate resin treatment, an absorption resin treatment, an addition of absorbent and so on are used in combination.
However, in the above processes, the content of Si is only reduced to about 1 ppb order. Therefore, an aqueous hydrogen peroxide solution purified by conventional processes could hardly be used in the field wherein a high purity is required such as in the field of the electronic industry. In the conventional processes of purification described above, there was another problem that the reproducibility of removal level of silicon oxide impurities was low and an aqueous hydrogen peroxide solution could not be efficiently purified.
Under these circumstances, the present inventors conducted extensive studies to solve the problems described above. As the result of the studies, it was discovered that there exists both soluble silica and insoluble silica (suspended particles or colloid) in the silicon oxide impurities contained in an aqueous hydrogen peroxide solution. It was also discovered that, when insoluble silica is filtered out with a precision filter after adding a flocculating agent to an aqueous hydrogen peroxide solution and soluble silica is further removed by using an anion exchange resin changed into a fluoride ion form by at least one fluoride compound which contains 0.05% by weight or less of SiF6 and is selected from the group consisting of sodium fluoride, potassium fluoride and ammonium fluoride, the concentration of Si in an aqueous hydrogen peroxide solution could be reduced up to ppt order (1/1012), and the reproducibility of removing silicon oxide impurities level was very high. The present invention has been completed on the basis of these findings.
The above Japanese Patent Application Laid-Open Publication No. 9(1997)-1428 12 also discloses a process for removing silicon oxide impurities by bringing an aqueous hydrogen peroxide solution containing silicon oxide impurities into contact with an anion exchange resin in the fluoride ion form. Usually, silicon compounds such as SiF6 are contained in high amounts as impurities in sodium fluoride, potassium fluoride, ammonium fluoride and the like (hereinafter called xe2x80x9cregenerantxe2x80x9d) which are used when changing an anion exchange resin into the fluoride ion form. As a result of the present inventors"" further studies, it was discovered that such silicon compound impurities in the regenerant could hardly be removed from an ion exchange resin to a sufficient degree and therefore, silicon compound impurities derived from the regenerant might be mixed into an aqueous hydrogen peroxide solution when an aqueous hydrogen peroxide solution was brought into contact with the ion exchange resin, which caused the inferior reproducibility of the concentration of Si in a purified aqueous hydrogen peroxide solution. On the other hand, when the regenerant containing a controlled amount of SiF6 to a certain level or less is used in the present invention, silicon compound impurities do not remain in an ion exchange resin, thereby silicon oxide impurities can be removed to an extremely high degree and it is also possible to realize a higher reproducibility of the removal level.
An object of the present invention is to provide a process for producing a highly purified aqueous hydrogen peroxide solution from which silicon oxide impurities are removed to a minimum by purifying an aqueous hydrogen peroxide solution containing silicon oxide impurities.
The present invention provides a process for producing a highly purified aqueous hydrogen peroxide solution by removing silicon oxide impurities contained in an aqueous hydrogen peroxide solution, which comprises:
adding a flocculating agent to an aqueous hydrogen peroxide solution containing silicon oxide impurities, and filtering out impurities of solid content contained in the aqueous hydrogen peroxide solution with a precision filter, thereafter
bringing the aqueous hydrogen peroxide solution obtained as above into contact with an anion exchange resin in a fluoride form changed by at least one fluoride compound which contains 0.05% by weight or less of SiF6 and is selected from the group consisting of sodium fluoride, potassium fluoride and ammonium fluoride.
Thus, a flocculating agent is added to an aqueous hydrogen peroxide solution in advance, then insoluble silica is removed by filtration using a precision filter. Subsequently, soluble silica is removed by using an anion exchange resin in a fluoride form changed by a fluoride compound wherein the amount of SiF6 is controlled to a certain amount or less As a result, it becomes possible to remove the silicon oxide impurities contained in an aqueous hydrogen peroxide solution up to ppt order (1/1012). Furthermore, the present process realizes the higher reproducibility of removing such silicon oxide impurities.
The present invention also provides a process for producing a purified aqueous hydrogen peroxide solution by removing silicon oxide impurities contained in an aqueous hydrogen peroxide solution to a minimum, which comprises:
adding a flocculating agent to an aqueous hydrogen peroxide solution containing silicon oxide impurities, and filtering out impurities of solid content contained in the aqueous hydrogen peroxide solution with a precision filter, thereafter
bringing the obtained aqueous hydrogen peroxide solution into contact with:
(i) an H+ cation exchange resin, then
(ii) an anion exchange resin changed in a fluoride form by at least one fluoride compound which contains 0.05% by weight or less of SiF6 and is selected from the group consisting of sodium fluoride, potassium fluoride and ammonium fluoride, then
(iii) an anion exchange resin in the carbonate form or bicarbonate form, thereafter
(iv) a cation exchange resin in H+ form.
In the present invention, it is preferred that the concentration of hydrogen peroxide contained in an aqueous hydrogen peroxide solution is 40 to 70% by weight. Especially, silicon oxide impurities can be removed with efficiency when the aqueous hydrogen peroxide solution having such concentration is used.
It is preferred that a flocculating agent is at least one phosphorus compound selected from the group consisting of phosphoric acid, polyphosphoric, disodium dihydrogen pyrophosphate, aminotri (methylenephosphonic acid) and the salt thereof, and ethylenediaminetetra (methylenephosphonic acid) and the salt thereof. It is preferred that the said phosphorus compound be added in an amount that the atomic ratio (Si/P) of silicon oxide impurities contained in an aqueous hydrogen peroxide solution in terms of silicon atom to the phosphorus compound in terms of phosphorus atom is 0.0001 or less.
It is preferred that the average pore diameter of the precision filter used in the present invention be 0.2 xcexcm or less.